This invention relates to a process for making copolymers of uniform molecular weight. In particular, it relates to the preparation of a high solids, low molecular weight, copolymer having a narrow weight distribution.
Conventional solvent based industrial finishes and coatings have presented many problems to date. Solvents can pollute the workplace and environment; they are readily ignited, they are toxic, they are expensive, they lower the quality of the finish and they can color an otherwise clear finish. As a replacement for these solvent-based finishes, the trend in the polymer industry has been toward high solids, liquid coatings. Such coatings typically have a solids content of at least about 70% (non-volatiles).
High solids coatings offer significant advantages over conventional, solvent-thinned coatings. They do not pollute the air; they reduce or eliminate exudation of fumes in use; they reduce energy requirements for their preparation in terms of material, energy expended and labor and, unlike solvent-based systems, they do not present significant fire and toxicity problems. High solids coatings also provide substantial advantages over other high solids liquids, such as solventless, waterborne, powder and non-aqueous dispersion systems and they also offer a better balance of properties.
Perhaps the most difficult problem in preparing and utilizing high solids coatings is selection and control of viscosity. It is recognized that to control viscosity in the preferable range from about 0.1 to 5 poises, it is desirable to employ low molecular weight resins or oligomers, alone or in combination with a reaction solvent. Higher molecular weight resins are usually too viscous to be employed for high solids applications. Conventionally prepared low molecular weight resins (Mn-500 to 6000, especially 1000 to 3000) are subject to several significant defects.
To obtain resins of low viscosity and good overall application performance, it has been found necessary to make resins having very narrow molecular weight distributions, Takahashi, Recent Advances In High Solids Coatings, Polm. Plast. Technol. Eng. 15(1), pp. 1, 10 (1980). It has been postulated that the presence of high molecular weight polymer fractions dominates the viscosity characteristics of a high solids, low molecular weight resin. The relative presence or absence of high molecular weight fractions is indicated by the polydispersity ratio and the distribution index.
The polydispersity ratio (Mw/Mn, W/N or ratio of weight average molecular weight to number average molecular weight) is of vital importance to scientists in this field. Products having the same average molecular weight, but having a different molecular polydispersity possess different solution viscosities. The product with the higher polydispersity always has a higher solution viscosity, because high molecular weight fractions make a significantly greater contribution toward viscosity than low molecular weight fractions.
There is a another molecular weight measure known as the sedimentation average molecular weight, Mz. In relative terms, the Mn&lt;Mw&lt;Mz. If only one molecular species is present then Mn=Mw=Mz. However, this theoretical representation is not the case with polymers made by free radical processes.
Mz is a rather specific measure of the number of molecules in the higher weight fractions of the molecular weight range. The distribution index or ratio of (Mz/Mn or Z/N) is a key measure of the range of molecular weight distribution for a given resin, and provides an indication of the presence or absence of higher weight fractions. Products with a higher distribution index will exhibit higher solution viscosities and less desirable application properties. Contemporary industry standards require that a process to prepare resins suitable for high solids systems have sufficient flexibility to selectively increase or decrease the molecular weight of the desired product and its polydispersity and distribution ratios in accordance with market requirements.
In addition, products containing undue quantities of very low molecular weight fractions (dimers, trimers, etc.) can exhibit number average molecular weights (Mn) skewed to be nonreflective of the properties of the product and can introduce substandard properties to the product. Very low molecular weight fractions, such as dimers, trimers, and other oligomers, can be quite nonuniform or heterogeneous, when compared to the desired product, especially if a terpolymer or tetrapolymer is prepared.
Applications for high solids coating include coatings and finishes for cans, coils, fabrics, vinyls, papers, autos, furniture, magnet wire, appliances, metal parts, wood panels and floors. Other typical applications for such coatings are as paints, inks, adhesives, tackifiers and dispersants. Such applications can require that copolymers be formed from hard monomers, soft monomers, acid monomers and/or monomers with other crosslinkable functionalities.
Attempts have been made to prepare high solids, low molecular weight acrylic (co)polymers in the 500 to 6,000 Mn range, due to the valuable advantages acrylics afford. Their relatively low cost, clear color, good outdoor durability, varying chemical resistances and good thermal stability are just some of the benefits attributed to acrylics. No process has been entirely successful in preparing high yields of a broad spectrum of high solids, low molecular weight acrylic polymer products having a narrow molecular weight distribution and good color which is of sufficient low viscosity for practical use.
An anionic process for making certain specific relatively narrow molecular weight acrylic oligomers with a Mn of 600 to 5,000 has been proposed as illustrated in U.S. Pat. No. 4,064,161. The polydispersity is said to be from 1.1 to 3. This anionic process presents distinct disadvantages, among them are: substantial residual levels of initiator fragments, the inability to copolymerize styrenic type monomers with the acrylic monomers (as noted in U.S. Pat. No. 4,137,389) and the inability to copolymerize oxyalkyl esters of acrylic or methacrylic acid. Further, a hydrolysis or transesterification step is required to obtain hydroxyl functional group containing oligomers for crosslinking purposes. This anionic process is also apparently not able to copolymerize an acrylic acid ester monomer with a methacrylic acid ester monomer.
Conventional free radical initiated processes for preparing low molecular weight acrylic copolymers have exhibited various defects and deficiencies. U.S. Pat. No. 3,028,367 proposed the use of organic thiol compounds for this purpose. These thiol produced products generally have offensive odors, varying color stability and poor outdoor weatherability. Further, the use of high levels of thiol compounds is required, which significantly effects the backbone composition of the polymer formed. U.S. Pat. No. 3,080,348 has suggested that the molecular weight of styrene-acrylate systems may be reduced by increasing reaction temperatures. However, this patent is said not to involve efforts to prepare low molecular weight polymers in the range from 500 to 6,000 as noted by U.S. Pat. No. 4,075,242.
U.S. Pat. No. 4,276,432 describes a process for making acrylic and/or styrenic copolymers with an Mn (as described by vapor phase osmometry) of 750 to 5,000. Reaction solvent is required at addition levels of 40 to 70% by weight of monomers. Long reaction times are employed from 1 to 10 hours. The excessive solvent stripping operation required due to the high levels of solvent employed in the process and the long feed times tend to make this process inefficient in terms of labor and capital expended, unduly time consuming and energy inefficient. The use of excessive amounts of inflammable, toxic and polymer contaminating solvent is a major problem.
Previously, styrene monomer has been homopolymerized to form high molecular weight polymers from 20,000 to 100,000 average molecular weight (Mw) in a continuous mass polymerization process without solvents, catalytic initiators and molecular weight regulators, as disclosed in U.S. Pat. Nos. 2,496,653 and 3,859,268. It has been generally recognized that at temperatures above about 200.degree. C., thermally initiated styrene polymerization produces an undesired molecular weight fraction (dimers, trimers, etc.) causing a wide range of molecular weight and having a high polydispersity (Mw/Mn).
It has been disclosed in U.S. Pat. No. 4,117,235 that batches of an acrylic monomer can be thermally polymerized in sealed glass tubes at temperatures from 230.degree. C.-280.degree. C. to provide an acrylate polymer with a number average molecular weight of less than about 5,000, in the presence or absence of a chain transfer agent or solvent. Excessively long reaction times are proposed of 16-18 hours. The process is conducted as a batch process with a bulk monomer charge and a subsequent long term cook at the reaction temperatures.
U.S. Pat. No. 3,979,352 discloses styrene-acrylic copolymers having an Mn said to be from 600 to 4,000. The process for preparing the copolymers is conducted in a heated tube. No polydispersities or distribution indexes are provided.
In order to provide clear, vinylic copolymers, such as styrene-acrylic copolymers, of high solids content, narrow molecular weight distribution and low solution viscosity, the art has long sought a fast, efficient, high yielding process capable of selectively producing a wide spectrum of hard, soft, alkali-soluble or thermosetting copolymers, which is safe, energy efficient and capable of using existing equipment without undue modifications required for long term, ultra-high temperature operation.
It has been suggested that a continuous bulk polymerization process would be extremely advantageous to provide acrylic copolymers in terms of cost, quality of product and stability. It is understood that solution-type batch processes employing large quantities of solvent and initiator are unsatisfactory, since too many impurities, including solvent, remain in the polymer, the quality of produced polymer is low and efficiency is low, as reported in U.S. Pat. No. 4,328,327. The continuous process proposed therein, however, employs reaction residence times up to 10 hours. It is also noted that when purity is critical, it is advisable not to use a polymerization initiator. Reaction temperatures are said to be below about 160.degree. C.
Accordingly, the art has sought, a continuous bulk polymerization process capable of selectively providing high yields of high purity, low molecular weight vinylic polymers suitable for high solids applications. The term "vinylic polymers" refers to the addition polymer formed by polymerizing vinylic monomers. The vinylic polymers sought should exhibit a narrow molecular weight distribution, low solution viscosity, low dimers and trimers content, low volatiles content, and good color. The process should be energy efficient and adapted for use with conventional equipment.